Working with Cassini RADAR


Instrument Overview

Cassini's microwave radio broadcasts are able to penetrate Titan's atmosphere. The radar observations help to determine the topography and surface properties. The specific goals of CSS-Radar is to determine existence of oceans on Titan and their distribution. Radar is also used to investigate the geologic features and topography of the solid surface of Titan.

Technical Details

Radar Modes

The Cassini Radar instrument operates in both passive (radiometer) and active (altimeter, SAR imaging, scatterometer) modes.

  • Synthetic Aperture Radar (SAR):
    An active mode, in which Cassini can build up images of the surface morphology.

    SAR images may be the sharpest that Cassini can achieve on the surface of Titan.

  • Altimetry:
    An active mode, in which Cassini pings a radio signal at Titan's surface and waits for the echo to return.

    After the effects of the spacecraft's forward motion and Titan's overall spherical shape are removed from the data,
    scientists can produce an altimetric profile along Cassini's track over Titan, showing the shape of the landscape.

  • Scatterometry:
    An active, real-aperature mode that produces regional-scale backscatter images across large areas of the
    surface. Scatterometry data are important complement to radiometry data with it's independent constraint
    on surface roughness.

  • Radiometry:
    A passive mode, in which Cassini points at a target and "listens" for radio energy emanating from Titan.

    Radiometry roughly correlates with temperature, and radiometry results are often described as "brightness temperatures" of surfaces.

ISIS3 Processing of PDS Radar Products

BIDRs (Basic Image Data Records)

  • BIDRs are the Single Pass, calibrated and gridded Synthetic Aperture Radar (SAR) data
  • Publically released in PDS/RDR format (Recognized in ISIS3 as a Level2 Map Projected product).
  • Produced in an oblique cylindrical map projection (coordinate system)

Data Ingestion

  • The PDS BIDR's are compressed and have a .ZIP file extention
  • The PDS/RDR BIDR product contains map projection keyword labels that are required by ISIS3.
  • The pds2isis application is used to ingest into ISIS3.
    • pds2isis translates the Oblique Cylindrical projection mapping parameters to the 'Mapping' Group labels of the ISIS3 output file.


Download the image and label files:

Search the PDS web site in "CORADR_0045" folder for the files listed below:
Find "BIBQH22N068_D045_T003S01_V02.ZIP" and "BIBQH22N068_D045_T003S01_V02.LBL"
on the PDS web site PDS IMAGE NODE and copy to local work area

Decompress the PDS File:

unzip BIBQH22N068_D045_T003S01_V02.ZIP

Ingest the PDS/RDR IMG file into ISIS3:

 pds2isis from=BIBQH22N068_D045_T003S01_V02.IMG to=BIBQH22N068_D045_T003S01_V02.cub

Reproject the Oblique Cylindrical

The BIDR's are in oblique cylindrical projection and each one has its own unique mapping parameters, so it is difficult to visualize where north is on the images. Converting the file to a different map projection helps with identifying the image coordinates. If the images are to be mosaicked they need to be reprojected to a common set of map parameters (map scale, center latitude, etc). See Learning About Map Projections.

There are two options, use the current mapping information and only modify the map projection, or redefine the appropriate mapping parameters to mosaic the images together.

Example 1: Convert to a different projection with map2map file for visual inspection:

 map2map from=BIBQH51S121_D177_T049S01_V02.cub to=BIBQH51S121_D177_T049S01_V02_simp.cub

Example 2: Define map parameters for mosaicking:

Refer to Learning About Map Projections for the fundamentals on defining a map in ISIS3.

Run maptemplate to redefine some mapping parameters in order to mosaic the reprojected images. The center latitude, center longitude, and map resolution must match. The user will also need to select the longitude domain (180 or 360).

Note: ISIS3 defaults to a Positive Longitude East direction, the map template file can be used to specify Positive Longitude West

 maptemplate projection=simplecylindrical clon=0 
 targopt=user targetname=Titan londir=positivewest londom=180

The program produces a PVL file containing the following:

 Group = Mapping
  ProjectionName     = SimpleCylindrical
  CenterLongitude    = 0.0
  TargetName         = Titan
  EquatorialRadius   = 2575000.0 <meters>
  PolarRadius        = 2575000.0 <meters>
  LatitudeType       = Planetocentric
  LongitudeDirection = PositiveWest
  LongitudeDomain    = 180

In this example, reproject the file to a simple cylindrical projection with map2map:

 map2map from=BIBQH51S121_D177_T049S01_V02.cub to=BIBQH51S121_D177_T049S01_V02_simp.cub

Occasionally map2map will fail if it cannot resolve the default latitude and longitude extents of the input file. The error message reported follows:

 Error message:
  Group = Error
    Program = map2map
    Code    = 1
    Message = "Unable to determine the correct
               [MinimumLongitude,MaximumLongitude]. Please specify these values
               in the [MINLON,MAXLON] parameters"
    File    = map2map.cpp
    Line    = 304

In most cases, the error is due to the longitude range falling outside of the longitude domain that was selected. For example, the 180 longitude domain was chosen but the longitude range for this file goes beyond 180. Since the map template does not contain the image boundary, the values in the labels are used. For this case, the latitude and longitude extents need to be entered by the user in order to remap the pixels to the correct locations in a 180 longitude domain. The values entered must be appropriate for the longitude domain selected.

  Group = Mapping
    ProjectionName     = ObliqueCylindrical
    TargetName         = Titan
    EquatorialRadius   = 2575000.0 <meters>
    PolarRadius        = 2575000.0 <meters>
    LatitudeType       = Planetocentric
    LongitudeDirection = PositiveWest
    LongitudeDomain    = 360
    **MinimumLatitude    = -89.99653831**
    **MaximumLatitude    = -10.56685759**
    **MinimumLongitude   = 0.0**
    **MaximumLongitude   = 360.0**
    UpperLeftCornerX   = -2067342.51008 <meters>
    UpperLeftCornerY   = 4449280.61952 <meters>
    PixelResolution    = 351.11116 <meters/pixel>
    Scale              = 128.0 <pixels/degree>
    Rotation           = 90.0
    PoleLatitude       = -15.827261 <DEG>
    PoleLongitude      = 342.714697
    PoleRotation       = 43.599373 <DEG>
    XAxisVector        = (-0.39349534, 0.59978022, -0.69672456)
    YAxisVector        = (-0.03558647, 0.74735851, 0.66346730)
    ZAxisVector        = (0.91863759, 0.28586526, -0.27273803)

Look at the latitude and longitude extents of the file, asterisk-marked values above, to help determine what values to enter. If the longitude range goes from 0 to 360 and the longitude domain selected is 180, then use minlon=-180 and maxlon=180. The only requirement is there be sufficient work space available to generate a large file.

  map2map from=BIBQH51S121_D177_T049S01_V02.cub to=BIBQH51S121_D177_T049S01_V02_simp.cub minlat=-90.0 maxlat=-10.5 minlon=-180 maxlon=180

Crop out only the portion of valid data (non-NULL) in the file to minimize disk space usage:

  cropspecial from=BIBQH51S121_D177_T049S01_V02_simp.cub 

Display the file to view the result:

 qview BIBQH22N068_D045_T003S01_V02.cub

BODPs (Burst Ordered Data Products)

ISIS3 currently does not support processing of these products

  • Data files include engineering telemetry, radar operational parameters, raw echo data, instrument viewing geometry, and calibrated science data
  • Level1/EDR PDS type format
  • Three different record formats
    • Short Burst Data Record (SBDR)
    • Long Burst Data Record (LBDR)
    • Altimeter Burst Data Record (ABDR)

DMP (Digital Map Products)

The following maps are currently being generated by Astrogeology (ISIS2) under the direction of Randy Kirk.

  • PRDR - Pass Radiometry Data Record
    A global map of Titan in Equirectangular (cylindrical) projection containing in gridded form the brightness temperature of the surface based on one sequence obtained by one Titan flyby pass. PRDR's are provided for the scatterometry, altimetry, and SAR sequences as well as for radiometry-only scans.

  • PSDR - Pass Scatterometry Data Record
    The PSDR resembles the PRDR in format and also contains gridded data for a single sequence of one Titan flyby. The primary mapped quantity in the PSDR is the backscatter cross-section.

  • GRDR - Global Radiometry Data Record
    The GRDR is a mosaic of gridded radiometric brightness temperature data (corrected to normal emission) assembled from the complete set of individual PRDR products.

  • GSDR - Global Scatterometry Data Record
    The GSDR is a mosaic of backscatter cross-section data (corrected to a reference incidence angle) assembled from the PSDR's.

  • GTDR - Global Topography Data Record
    The GTDR is a mosaic of absolute elevation values obtained on multiple Titan flybys. The GTDR contains both altimetric data and 'SAR topography' elevations derived by monopulse analysis of overlapping beams in the SAR images.

  • MIDR - Mosaicked Image Data Record
    Each MIDR is a mosaic of synthetic aperture radar (SAR) image data assembled from the BIDR's obtained on multiple Titan passes. The MIDR mosaics will be global coverage divided into separate quad boundaries.

  • RIDR - Repeat Image Data Record
    The RIDR product is designed to facilitate comparison of overlapping (repeat) SAR image coverage. A RIDR will be produced for every SAR image (BIDR), each image will be mapped into the same quad boundaries as defined for the MIDR products.

  • DTM - Digital Topographic Model [Generated using SOCET Set: trademark(BAE Systems)]
    Where suitable data are available, DTMs will be generated. Each DTM is a gridded data product containing absolute or relative elevation values.